43 CFR 3832.31 - What is a mill site?
A mill site is a location of nonmineral land not contiguous to a vein or lode that you can use for activities reasonably incident to mineral development on, or production from, the unpatented or patented lode or placer claim with which it is associated.
(a) A dependent mill site is used for activities that support a particular patented or unpatented lode or placer mining claim or group of mining claims.
(b) An independent or custom mill site -
(1) Is not dependent on a particular mining claim but provides milling or reduction processing for nearby lode mines or a lode mining district;
(2) Is used to mill, process, and reduce either -
(i) Ores for other miners on a contractual basis; or
(ii) Ores that are purchased by the independent or custom mill site owner.
(3) You may not have a custom or independent mill site for processing materials from placer mining claims.